Former star of American musicals in a main role? (6)
I believe the answer is:
merman
I believe this clue is a double definition.
'former star of american musicals' is the first definition.
(Ethel Merman)
'a main role?' is the second definition.
('the main' was a historic name for the sea)
'in' acts as a link.
(Other definitions for merman that I've seen before include "Half man, half fish" , "Fabulous creature" , "Ethel --, singer" , "fishy character" .)